Although the present invention is illustrated herein in relation to the synthesis of sucralose, the method disclosed can be applied to any situation where it is desirable to obtain purified DMF from a mixture containing DMF and HCl.
Methods for producing sucralose intermediates and sucralose from a stream comprising a sucrose-6-acylate in a reaction vehicle are known. For example, EP 0409549 discloses a process for the chlorination of a sucrose-6-acylate in a tertiary amide reaction vehicle to produce a sucralose-6-acylate, such as sucralose-6-acetate. A large excess of an acid chloride, such as phosgene, is used as the chlorination agent in this process. Following the chlorination reaction, the excess chlorinating agent is quenched using a suitable base, thereby forming the chloride salt of the base. The resulting product stream thus comprises a sucralose-6-acylate, the tertiary amide reaction vehicle, water, and salts.
Methods for preparing sucrose-6-acylate starting materials for chlorination to sucralose-6-acylate are known, for example, in U.S. Pat. No. 4,950,746; U.S. Pat. No. 4,889,928; U.S. Pat. No. 5,023,329; U.S. Pat. No. 5,089,608; U.S. Pat. No. 5,034,551; U.S. Pat. No. 5,470,969; U.S. Pat. No. 5,440,026; U.S. Pat. No. 6,939,962; and US 2007-0227897.
Other methods of chlorinating sucrose-6-acylate to give sucralose-6-acylate are known, for example, in U.S. Pat. No. 4,380,476; US 2006-0205936; U.S. Pat. No. 7,932,380; and US 2007-0100139.
It is often desirable to recover the reaction solvent after completion of a reaction to minimise waste, or to reuse the solvent. Particularly in sucralose synthesis, it is desirable to recover DMF from the reaction mixture after chlorination of a sucrose-6-acylate to a sucralose-6-acylate for reuse.
U.S. Pat. No. 5,530,106 discloses a method of removing DMF from the product mixture after quench of the chlorination reaction of a sucrose-6-acylate to a sucralose-6-acylate. The process comprises removing the DMF by steam distillation.
U.S. Pat. No. 8,530,643 discloses a method of removing DMF from the reaction mixture in the chlorination of a sucrose-6-acylate to a sucralose-6-acylate, before quench of the reaction, by distillation, to afford DMF. This DMF is contaminated with HCl. According to U.S. Pat. No. 8,530,643, the HCl may be removed by treating the HCl-contaminated DMF with a base. The mixture may be treated with aqueous solutions of base, such as sodium hydroxide or sodium carbonate. Alternatively, the mixture may be treated with bases under dry conditions to keep water levels low by using sodium carbonate, calcium hydroxide, sodium hydroxide or ammonia.
Accordingly, there is a need for methods of obtaining purified DMF from mixtures of DMF and HCl.